The scope of this study was to modify the natural clinoptilolite zeolite available locally (Akita Prefecture, Japan) for its ammonium ions retention capacity. The natural clinoptilolite was modified chemically and mechanically with changing time duration of sodium hydroxide treatment and ball to powder mass ratio in wet ball milling, respectively. The ammonium ions retention capacity of thus obtained modified clinoptilolites were found to sharply increase with either increasing alkaline metal cations content or increasing specific surface area (decreasing particle size) of the clinoptilolite. The main mechanism of ammonium ions retention is ion exchange and Na+ ions were observed to be more easily exchanged for ammonium ions. The sorption isotherms were good fit to the Langmuir model in the cases of natural and chemically modified clinoptilolites while Freundlich model was favorable in the case of mechanically modified clinoptilolites. The maximum NH4+ retention capacities of natural clinoptilolite (NZeo), clinoptilolite treated with NaOH solution for 72 h (Zeo-72) and wet-milled clinoptilolite (WM-50) according to Langmuir model were 0.89, 1.15 and 1.39 mmol/g, respectively. The overall reaction is pseudo-second-order with rate constant of 3.6 x 10(-2) dm(3) g/(mmol min). It was possible to enhance the NH4+ retention capacity of natural clinoptilolite just by decreasing particle size without incorporating any further exchangeable cations within the framework of zeolite. (C) 2009 Elsevier B.V. All rights reserved.